1. Technical Field
The invention relates to a computer tomograph with non-contacting transmission of energy. In this, the transmission of the energy needed by an X-ray tube is effected without contact between a stationary current supply and an X-ray tube disposed to be rotatable. Simultaneously, current may be supplied to other consumers such as detectors or data processing systems on the rotating part.
2. Description of the Prior Art
With conventional computer tomographs a transmission of electrical energy between a stationary current supply and a rotating part is effected by means of mechanical slip-ring systems. In this, a brush, preferably of a carbon material, slides along a slide track, for example of brass. Disadvantages of this arrangement are a low lifetime, regular servicing intervals in which the brushes must be changed, and contamination by abrasion of the carbons.
An improvement is disclosed, for example, in U.S. Pat. No. 4,912,735. In this, the computer tomograph system is configured to be a rotating data transmission device. A primary winding, fed by a primarily disposed alternating current source, is located on a stationary side. A secondary winding is mounted opposite to this on a rotating side. For better coupling between the primary winding and the secondary winding, these are surrounded by rotationally symmetrical cores of magnetically soft materials. However, this device is not suitable for transmitting high power in a range of 100 kilowatts, as would be necessary to energize modern X-ray tubes. This is because the transmission device has a high stray inductance owing to unavoidable air gaps between the stationary and the rotating side. Electrically this acts like a series inductance and thus represents a high series impedance for the current to be transmitted, which limits the transmittable power.
Another improvement is disclosed in U.S. Pat. No. 5,608,771. Here the stray inductance of the transmission is supplemented by another inductance and a capacity to form a resonance circuit. At the same time, the high voltage transmitter is connected directly to the secondary winding of the rotating part. With this arrangement, a certain stray inductance can now be tolerated. However, a very high coupling factor of the rotating data transmission device is needed, because otherwise too large a part of the current would flow through the primary winding of the transmission device as a reactive current. Moreover, with high stray inductance a reasonable matching to the high voltage transmitter would hardly be possible.
A disadvantage of both above-mentioned arrangements is a high outlay of material in the form of costly, highly-permeable ferromagnetic materials. Thus, with typical dimensioning, a few 100 kilograms of iron or ferrite material would be needed. These also would substantially increase the total weight of the computer tomograph. Especially troublesome is the large mass of the rotating part, because this also requires a design of bearing means of suitable strength. Another disadvantage results from the high demands made on mechanical tolerances during rotation between the rotating and the stationary part. Thus, an air gap between the primary side in the stationary part, and the secondary side in the rotating part, should be ideally within a range of a few tenths of a millimeter. However, typical tolerances that can be attained in computer tomographs are larger by almost one order of magnitude. Particularly critical is an operation of a rotating part that is inclined to a horizontal axis, because here the rotating part tilts from its normal position with respect to the stationary part.